Sign And Promise

Jam tunc propter peregrinationem futuram, ne commisceretur semen ejus inter gentres, datur ei circumcisio in signum his verbis: Circumcideter in bovis omne masculum. Infans octo dierum circumcidetur in vobis, tam vernaculus quam emptitius. Masculus autem, cujus praeputii caro circumcisa non fuerit, delebitur anima illa de populo suo, quia pactum meum irritum fecit. Sed et Sarai non vocabitur Sarai, sed Sara, et dabo tibi ex ea filium, et benedicam illum, et erit in nationes, et reges gentium ex eo erunt. Hic jam declaratur promissio de vocatione gentium in Isaac filio promissionis, quo significatur gratia, non natura, quia de sene patre et sterili matre. Et quia hoc non per generationem, quae est in Ismaele, sed per regenerationem futurum erat, ideo imperata est circumcisio, quod de Sara promittitur filius typo Ecclesiae, non quando Ismael, qui typum gerit Judaeorum. Quid enim aliud circumcisio significat, nisi renovatam naturam per baptsimum post expoliationem veteris hominis? Et quid est octavus dies, nisi Christus, qui hebdomada completa, hoc est, post Sabbatum resurrexit? Sanctus Isidorus Hispalensis,Mysticorum Expositiones Sacramentorum Seu Quaestiones In Vetus Testamentum, In Genesim, Caput XIII Source: Migne PL 83.242b-c

Then because of future wanderings, lest his mix his seed with the nations, circumcision is given to Abraham as a sign, with these words: 'Let every male be circumcised in the foreskin. Let the child be circumcised on the eighth day among you, he who is house born and he who is bought. The male who does not have his flesh circumcised wil have his soul removed from among his people, because he has violated my covenant.' 2 'But even Sarai will not called Sarai but Sarah, princess, and I shall give to you a son from her, and I shall bless him and he shall be among the nations, and the kings of the peoples shall come from him.' 2 Here is declared the promise concerning the calling of the nations in Isaac, the son of promise, by which is signified grace, not nature, because he came from an aged father and a barren mother. And because this is not through generation, which is in Ismael, but for the regeneration that is to come, therefore circumcision is commanded, that from Sarah is the promised son who is a type of the Church, and not from Ismael who bears the type of the Jews. What indeed does circumcision signify, unless the renewel of nature through baptism after the casting off of the old man? 3 And what is the eighth day, unless Christ who completes the seven days, that is, He who after the Sabbath is resurrected? Saint Isidore of Seville, Expositions of Sacred Mysteries or Questions on the Old Testament, On Genesis, Chap 13 1 Gen 17.10-12 2 Gen 17.15 3 Ephes 4.22